The present invention relates to power supplies for providing DC voltage from an AC source. More particularly, the invention relates to a power supply which is suitable for use in a lamp dimming module of the type which is used in microprocessor-based appliance and lamp control systems.
Regulated power supplies are fundamentally of two basic types, viz., analog or pass type and switching type. Examples of both types may be found in "Voltage Regulator Handbook", published by National Semiconductor Company, pages 7-3 and 7-4 (1980). The pass device supply provides DC current as requested by the load, and the switching type supply essentially switches between full on and full off conditions for varying times. However, both types of power supply can be operated at only a single AC input frequency, and the circuitry must be modified to accommodate use with a different frequency AC source. Even more importantly, neither type of power supply can operate with an intermittent AC source, i.e., one wherein the AC voltage is present for only a portion of each cycle of the AC wave.
This latter requirement is of particular importance in lamp dimming applications. An appliance control system which utilizes lamp dimming modules is disclosed in U.S. application Ser. No. 271,244 filed June 8, 1981, and assigned to the assignee of the present invention. Such a system includes a central control unit and a plurality of slave units each including a microprocessor and respectively plugged into outlet sockets of an AC line. Lamps or other appliances are respectively coupled to the slave units. Among other functions which can be performed by such a system, lamps plugged into remote slave units can, under remote control from the central control unit, be turned on or off or dimmed to any desired degree of brightness. The slave units of that system are so arranged that the lamp is connected in parallel with the slave modules so that the AC line voltage is always present at the module terminals, regardless of the condition of the lamp.
However, it has been found desirable in certain applications to arrange the slave units so that the lamp is connected in series with the module, one such application being where the slave unit is wired directly to the AC line in place of a wall switch control for the lamp. The slave unit typically includes a triac-type dimming circuit wherein the triac is fired into conduction by the microprocessor at a predetermined point during each half cycle of the AC voltage waveform, the triac remaining conductive for closing the lamp circuit for the remainder of that half cycle. The brightness of the lamp is determined by the proportion of each half cycle that the triac is conductive. When the triac is conductive and the lamp is on, virtually the entire AC voltage drop is across the lamp and substantially no AC voltage is available to the microprocessor power supply in the slave unit. The standard types of power supply will not operate properly when the AC source voltage is collapsed in mid-cycle in this manner.